This invention relates to a precision gimbal such as that used in missile seeker systems, and, more particularly, to the fabrication of such a multipiece gimbal.
In some types of missile systems, a seeker or sensor is placed in the nose of the missile, covered by a transparent window. The sensor may be a visible-light, infrared, or radar sensor. The sensor views a scene generally forward of the missile, and provides images of the scene to an automated image processor or to an operator.
Although the sensor field of view is generally directed forwardly of the axis of the missile, it is desirable for some types of missiles and sensors to have the capability to direct the sensor field of view in off-axis directions, up to 20 degrees or more from the missile axis. To provide this capability, the sensor is mounted on a motor-driven gimbal that permits the sensor to rotate about the missile axis and pivot about a bore having a transverse axis. This combination of motions permits the sensor to be pivoted to view any axial or off-axis scene, up to the permitted rotational angle of the gimbal.
The gimbal is provided as an investment casting having a base and two side pieces through which the transverse bore is formed. The gimbal is final machined to extremely precise dimensional tolerances in three critical areas. The height of the transverse bore from the base, the angular misorientation of the transverse bore, and the separation of the side pieces must be held to small tolerances, so that the gimbal angular position can be established accurately.
It is difficult to obtain and then retain these accurate dimensional tolerances. The complex shape of the gimbal requires specialized final machining steps that are not in themselves difficult to perform, but are difficult to perform to the required tolerances. The dimensional tolerances are additive to some extent. Perhaps even more significant, when viewed in side elevation, the gimbal is generally U-shaped, with a flat base and the side pieces extending upwardly at a right angle. When the investment casting is machined, the removal of metal causes the regions to relax so that previously established dimensions change. These changes in dimensions continue as the part is heated and cooled, due to residual stresses and other effects in the part. These changes are not large, but can exceed the critical tolerances unless great care is taken in the casting, heat treating, and final machining operations.
The result of the highly exact dimensional tolerances is that the gimbal is expensive to produce. The yield of parts is relatively low, because one small out-of-tolerance condition in the latter stages of the final machining operation may result in scrapping of the entire piece. The more dimensions that must be maintained critical in any part, the more difficult is the part to fabricate. In this case, the U-shaped gimbal is difficult and expensive to produce due to the high precision required and inherent effects such as the residual stresses.
There is therefore a need for an improved approach to the gimbal used in missile sensor systems. Any improved approach must provide the same capabilities as does the present gimbal, but desirably is less expensive to produce and is more stable in use. The present invention fulfills this need, and further provides related advantages.